1. Field of the Invention
The present invention relates to an inkjet head that sprays drops of ink on a recording medium, such as paper, to form an ink image, and also relates to a method for manufacturing the inkjet head.
2. Description of the Related Art
The following are examples of conventional technologies of the inkjet head.
(1) Japanese Patent Publication No. 2976479
Conventionally, a pressure-generating means that sprays drops of ink from a cavity in the inkjet head has been provided to each cavity, for example, by an adhesion process. However, according to Japanese Patent Publication No. 2976479, the pressure-generating means is provided on a silicon substrate by a process other than an adhesion process.
(2) Japanese Patent Laid-Open No. 07-276636
Japanese Patent Laid-Open No. 07-276636 defines the orientation of a crystal plane on a cavity wall in a method for forming the cavity by etching a silicon substrate.
Problem 1: In such conventional technologies, the shape of the cavity depends on the anisotropic etching of the single-crystal silicon. Since the etching, in turn, depends on the crystal structure of the single-crystal silicon, the shape of the cavity is limited by the crystal structure of the single-crystal silicon. In general, the cavity has a (111) face of the single-crystal silicon. The etching rate is low on the (111) face. Thus, the etching based on the orientation of the crystal plane produces a cavity wall that is not perpendicular to the silicon substrate, resulting in a lower cavity density.
The cavity wall is required to have an affinity for ink to prevent the deposition of air bubbles.
Problem 2: Conventionally, the etching of the silicon substrate has been performed by selective etching based on the difference in the concentration of doped p-type impurities. However, the selection ratio of the selective etching is several tens at the highest. Thus, when both a thin film portion of the substrate and the cavity portion are made of silicon, the thickness of the thin film portion may be poorly controlled and may vary. Japanese Patent Laid-Open No. 2002-234156 discloses a method using a silicon-on-insulator (SOI) substrate, in which a buried silicon oxide layer serves as an etch stop material. In alkaline etching, the etching rate of silicon oxide is less than one-thousandth of that of silicon and accordingly the selectivity is excellent.
However, in alkaline etching, heat treatment during the formation of a diaphragm or the subsequent formation of pressure-generating means or peripheral circuitry may cause precipitation of oxygen in the substrate. The precipitated oxide acts as a mask during the etching because of its low etching rate for an alkaline solution, and thus may cause nonuniform etching. Furthermore, such an oxide deposited on the diaphragm may cause nonuniform mechanical properties in the diaphragm, leading to fracture or crack of the diaphragm.
Problem 3: An SOI wafer is about 4 to 10 times as expensive as a single-crystal silicon wafer. In addition, when an SOI wafer having a thick thin-film layer is manufactured by lamination and polishing, variations in the thickness of the SOI layer, which are about ±0.5 μm, cause variations in the thickness of the thin film portion.